432 PHILOSOPHICAL TRANSACTIONS. [ANNO 1799. 



seems altogether to depend on the contraction and relaxation of the iris. When 

 the animal is alarmed or first disturbed, it naturally dilates the pupil, and the eye 

 glares; when it is appeased or composed, the pupil contracts, and the light in the 

 eye is no longer seen. The most material information that has been gained in this 

 investigation, is the transparent state of the retina in the eye during life; the opaque 

 membranous appearance which it puts on in the dead body not being natural to it, 

 but a change which takes place in consequence of death. This fact is almost all 

 that is necessary to explain the luminous appearance in the eyes of cats. 



That neither Baron Haller nor Fontana had an adequate idea of the transpa- 

 rency of the retina, will appear from the following expressions respecting it, taken 

 from their works : Haller describes it in the following words : "Membranam crassam 

 quidem, sed mollisimam, pellucidam utique, quando recens oculus inspicitur, ut 

 per earn sub aquis choroideam videas; tamen ex flavo subcineream."* So that, 

 though he calls it transparent, he says it is of a yellowish ash-colour. Fontana's 

 expressions are, *' Cette insensibilite de la retine, a la lumiere, en tant que 

 lumiere, derive- telle de ce que les nerfs sont encore trop gros, et ne sont pas bien 

 decouverts des tissus cellulaires ? ou de ce que la pulpe de la retine est trop 

 amoncelee, et empeche les rayons de lumiere d'arriver jusqu'a ces memes 

 nerfs V'-\" 



In considering the use of the lucid tapetum, it was an idea of the late Mr. 

 Hunter's, that the retina received a double stroke from the rays of light which 

 entered the eye; one in passing to the tapetum, the other in returning from it. 

 This very ingenious opinion had some difficulties opposed to it, while the retina 

 was supposed capable of obstructing the rays of light even in the smallest degree, 

 as they could not be equably transmitted, so as to affect every part of the mem- 

 brane alike. But the retina being ascertained to be absolutely transparent, these 

 objections are entirely removed, and there can be no doubt that the rays of light, 

 in those eyes which have a lucid tapetum, must remain on the retina as long again 

 as in the eyes of other animals; since the time required to strike on the tapetum, 

 and return, must be twice as much as is necessary for passing through the retina, 

 to reach the nigrum pigmentum, where they are lost. This may appear to be a 

 consideration of little consequence, as the velocity of light is so great, and the 

 continuance of impression necessary for distinct vision is that produced by a suc- 

 cessive flow of similar rays of light from the object; it may however be all that is 

 necessary for the purpose. 



The retina being found perfectly transparent, when the eye is examined in a 

 recent state, led to the eye that the internal structure of the optic nerve, when 

 examined in the same state, might also be transparent. To ascertain this point, 

 the following experiment was made: The posterior -£- of a cat's eye, while in a very 

 recent state, was immersed in a basin of water, and examined. The tapetum 

 appeared very bright, the retina not having acquired sufficient opacity to become 



* Elementa Pbysiologiae, torn. 5, p. 385.-Orig. + Sur le Venin de la Vipere, 1781, vol. 2, p.S19.-Orig. 



